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The Human Difference

We can embrace poetical reminders of our connection to the natural world, whether expressed as romantic effusions about oneness, or in the classical meter of Alexander Pope's heroic couplet:

All are but parts of one stupendous whole,

Whose body Nature is, and God the soul.

Yet, when we look into the eyes of an ape, our perception of undeniable affinity evokes an eerie fascination that we usually express as laughter or as fear. Our discomfort then increases when we confront the loss of former confidence in our separate and exalted creation ''a little lower than the angels . . . crowned . . . with glory and honor'' (Psalm 8), and must own the evolutionary alternative, with a key implication stated by Darwin himself (in ''The Descent of Man''): ''The difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind.''

We have generally tried to unite our intellectual duty to accept the established fact of evolutionary continuity with our continuing psychological need to see ourselves as separate and superior, by invoking one of our worst and oldest mental habits: dichotomization, or division into two opposite categories, usually with attributions of value expressed as good and bad or higher and lower. We therefore try to define a ''golden barrier,'' a firm criterion to mark an unbridgeable gap between the mentality and behavior of humans and all other creatures. We may have evolved from them, but at some point in our advance, we crossed a Rubicon that brooks no passage by any other species.

Thus, we have proposed many varied criteria -- and rejected them, one by one. We tried behavior -- the use of tools, and upon failure of this broad standard, the use of tools explicitly fashioned for particular tasks. (Chimps, after all, strip leaves off twigs, and then use the naked sticks for extracting termites out of nests.) And we considered distinctive mental attributes -- the existence of a moral sense, or the ability to form abstractions. All have failed as absolutes of human uniqueness (while a complex debate continues to surround the meaning and spread of language and its potential rudiments).

The development of ''culture'' -- defined as distinct and complex behavior originating in local populations and clearly passed by learning, rather than genetic predisposition -- has persisted as a favored candidate for a ''golden barrier'' to separate humans from animals, but must now be rejected as well. A study published in a recent issue of the journal Nature proves the existence of complex cultures in chimpanzees. This research proves that chimpanzees learn behaviors through observation and imitation and then pass them on to other chimpanzees. The study represents the cooperative effort of all major research groups engaged in the long-term study of particular groups of chimpanzees in the wild (with Jane Goodall's nearly 40-year study of the Gombe chimps as the flagship of these efforts).

Isolated examples of cultural transmission have long been known -- with local ''dialects'' of songbirds and the potato washing of macaques on a small Japanese island as classical cases. Such rudimentary examples scarcely qualified as arguments against a meaningful barrier between humans and animals. However, the chimpanzee study, summarizing 151 years of observation at seven field sites, found culturally determined, and often quite complex, differences among the sites for 39 behavioral patterns that must have originated in local groups and then spread by learning.

To cite just one example, contrasting the two best studied sites (Ms. Goodall's at Gombe and Toshisada Nishida's in the Mahale Mountains 170 kilometers away, with no recorded contact between the groups), the Mahale chimps clap two hands together over their head as part of the grooming ritual, while no Gombe chimp has ever so behaved. (Grooming itself may be genetically enjoined, but such capricious variations in explicit style must be culturally invented and transmitted.) In the commentary that accompanies the Nature article, Frans B. M. de Waal of the Yerkes Regional Primate Research Center in Atlanta summarized the entire study by writing, ''The evidence is overwhelming that chimpanzees have a remarkable ability to invent new customs and technologies, and that they pass these on socially rather than genetically.''

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The conventional commentary on such a conclusion would end here, leaving a far more important issue unaddressed. Why are we so surprised by such a finding? The new documentation is rich and decisive -- but why would anyone have doubted the existence of culture in chimps, given well-documented examples in other animals and our expanding knowledge of the far more sophisticated mental lives of chimpanzees?

Our surprise may teach us as much about ourselves as the new findings reveal about chimpanzees. For starters, the basic formulation of them vs. us, and the resulting search for a ''golden barrier,'' represents a deep fallacy of human thought. We need not fear Darwin's correct conclusion that we differ from other animals only in degree. A sufficient difference in quantity translates to what we call difference in quality ipso facto. A frozen pond is not the same object as a boiling pool -- and New York City does not represent a mere extension of the tree nests at Gombe.

In addition, evolution does provide a legitimate criterion of genuine and principled separation between Homo sapiens and any other species. But the true basis of distinction lies in topology and genealogy, not in any functional attribute marking our superiority. We are linked to chimpanzees (and more distantly to any other species) by complete chains of intermediate forms that proceed backward from our current state into the fossil record until the two lineages meet in a common ancestor. But all these intermediate forms are extinct, and the evolutionary gap between modern humans and chimps therefore stands as absolute and inviolate. In this crucial genealogical sense all humans share equal fellowship as members of Homo sapiens. In biological terms, with species defined by historical and genealogical connection, the most mentally deficient among us is as fully human as Einstein.

If we grasped this fundamental truth of evolution, we might finally make our peace with Alexander Pope's location of human nature on an ''isthmus of a middle state'' -- that is, between bestiality and mental transcendence.

We might also become comfortable with his incisive characterization of our peculiar status as ''the glory, jest, and riddle of the world.''

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A version of this op-ed appears in print on July 2, 1999, on Page A00017 of the National edition with the headline: The Human Difference. Today's Paper|Subscribe